The present invention relates to a scanning-type image display apparatus which, mainly, is mounted on the head of a user and allows the user to observe an image by scanning a light flux on an eye (retina) of the user.
A retina 1 scanning-type image display apparatus has been disclosed, for example, in U.S. Pat. No. 5,467,104, Japanese Patent Laid-Open No. 2004-347687, and U.S. Pat. No. 6,157,352. In those retinal scanning-type image display apparatuses, a scanning device and an ocular optical system are used to perform two-dimensional scanning of red light, green light, and blue light from a light source on a retina of an observer to allow observation of an image.
In those retinal scanning-type image display apparatuses, a scanning device having a small size and capable of fast scanning of light is used in order to realize a higher resolution. Since such a small scanning apparatus is used, a light beam for scanning needs to have a very small diameter.
This involves the problem of the likelihood of vignetting in images since the diameter of the light beam is also small in the exit pupil area in which the eye of an observer is placed. A method for increasing the diameter of the exit pupil is to dispose an exit pupil expander in an optical system of a retinal scanning-type image display apparatus as disclosed in U.S. Pat. No. 6,157,352. Specifically, an exit pupil expander such as a lens array and a diffusing plate is disposed close to the position of the intermediate image plane of the optical system to provide the divergent angle of a light flux emerging from the exit pupil expander that is larger than the divergent angle of the light flux entering the exit pupil expander, thereby increasing the diameter of the exit pupil.
When the exit pupil expander as disclosed in U.S. Pat. No. 6,157,352 is used, however, the following problem arises if the emergence angle of the light emerging from the exit pupil expander is significantly larger than the angle of the light entering the exit pupil expander. Specifically, although the diameter of an area (exit pupil) for allowing an observer to see an image is increased, the amount of light reaching the eyes of the observer is relatively small, presenting a darker image. In addition, the diffused light may serve as stray light to make it difficult to perform favorable image observation.
The diameter (width) of the exit pupil formed by the light emerging from the exit pupil expander in a certain direction is equal to that in a direction orthogonal thereto. Thus, when a rectangular image is displayed, a loss of light amount is not satisfactorily small in that optical system.